1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to contactors. The disclosed concept also relates to a method of operating electrical switching apparatus.
2. Background Information
Contactors are employed, for example and without limitation, in starter applications to switch on/off a load as well as to protect a load, such as a motor or other electrical device, from current overloads. Contactors are used as electrical switching apparatus and incorporate fixed and movable contacts that when closed, conduct electric power.
For example, three-pole, low voltage contactors have three contact assemblies, one contact assembly for each phase or pole of a three-phase electrical device. Each contact assembly can include, for example, a pair of stationary contacts and a moveable contact. One stationary contact is a line side contact and the other stationary contact is a load side contact. The moveable contact is controlled by an actuating assembly comprising an armature and magnet assembly, which is energized by a coil to move the moveable contact to form a bridge between the stationary contacts. When the moveable contact is engaged with both stationary contacts, current is allowed to travel from the power source or line to the load or electrical device. When the moveable contact is separated from the stationary contacts, an open circuit is created and the line and load are electrically isolated from one another.
Generally, a single coil is used to operate a common carrier for all three contact assemblies. As a result, the low voltage contactor is constructed such that whenever a fault condition or switch open command is received in any one pole or phase of the three-phase input, all the contact assemblies of the contactor are opened in unison. Simply, the contact assemblies are controlled as a group as opposed to being independently controlled.
Medium voltage contactors generally include air gap, insulating gas and vacuum varieties. For example, vacuum contactors interrupt an electrical arc within a vacuum.
A single-phase vacuum contactor, for example, includes a vacuum bottle having a suitable highly evacuated vacuum maintained therein, an operating mechanism, an alternating current (AC) power line terminal and a load terminal. For example, a fixed contact and a movable contact are contained within the vacuum bottle and are electrically connected to the line terminal and a movable bottle stem, respectively. The load terminal of the contactor is electrically connected by a shunt to the bottle stem which protrudes from the bottle. Movement of the bottle stem away from the bottle moves the movable contact away from the fixed contact and, thus, separates the contacts in an open position. The operating mechanism includes, for example, a T-shaped crossbar which is rotatable about a bearing, and a coil having an armature which is responsive to the coil and attached to the crossbar in order to rotate the crossbar. The T-shaped crossbar has a kickout arm and a pivot plate arm.
Examples of medium voltage or vacuum contactors including a number of poles are disclosed in U.S. Pat. Nos. 5,559,426; 4,559,511; 4,544,817; 4,504,808; 4,485,366; 4,479,042; and 4,247,745.
Known contactors employ a fixed opening time that is coordinated with main starter power fuses to ensure that the contactor does not interrupt a fault that is greater than its interrupting rating. For example, the opening time of a typical contactor is in the order of about 8 line cycles or about 130 milliseconds. The contactor waits this amount of time to open even if the fault current is below the contactor's interrupting rating.
As another example, some known contactors made by different manufacturers have interrupting capabilities of between 4400 amperes and 8500 amperes and employ a fixed or predetermined drop out time of between 2 and 20 line cycles.
There is room for improvement in electrical switching apparatus, such as contactors.
There is also room for improvement in methods of operating electrical switching apparatus.